Stainless bearing steels having high hardness levels (e.g., Rc 57-62) required for wear and fatigue resistance unfortunately suffer from limited fracture toughness. This is of particular concern in bearing applications requiring support of tensile stresses in the bearing as, for example, in the bearing races of the high speed fuel and oxidizer turbopumps of the main engine of the space shuttle. In these turbopumps, Type 440C stainless steel ball bearings/bearing races (hardness Rc 59) are used to support shafts rotating at 29,000 rpm at a temperature below minus 300.degree. F.
In addition to high loads and low temperatures, the turbopump bearings are also subjected to hostile lubrication conditions aggravate by the corrosiveness of the liquid oxygen supplied by the turbopump to the main engine. Corrosion, in particular stress corrosion cracking, of the bearings is thus an additional concern.
The Type 440C stainless steel bearings of the high speed fuel and oxidizer pumps were designed for a service life of 55 shuttle flights before replacement. The combination of low stress corrosion resistance and low fracture toughness (e.g., 22-23 KSI in..sup.1/2 at room temperature) of the Type 440C bearing material make bearing race cracking a serious concern. As a result, the bearings are now inspected and tested thoroughly after each shuttle flight and are replaced, if necessary. This inspection and premature replacement of the bearings has become a significant source of delay and expense between shuttle flights.
There is a need for a stainless steel having improved fracture toughness and corrosion resistance at a given high hardness level (e.g., at least Rc 59) needed for service as a bearing material in the aforementioned shuttle high speed fuel and oxidizer turbopumps as well as in other service applications where load, temperature and/or corrosion conditions require a combination of high hardness (e.g., at least Rc 57) for wear and fatigue resistance, fracture toughness, and corrosion resistance.